Can't bevel round cutouts in a cube
I opened the first of the attached files, StopTablet2.blend... the Bevel Modifier was before the Boolean Modifier so I changed that. And voilà, the object was beveled. And even more after disabling Clamp Overlap.
In the second file StopTablet3.blend you've set Limit Method of the Bevel Modifier to Weight, but only the edges on the top have bevel weights assigned (you can see it by the blue color). If you give a bevel weight to the other edges you want to bevel it works, at least if you disable Clamp Overlap again. Or use a different Limit Method like you did in the other file.